Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely implemented for large-scale energy storage applications. Various battery designs, including flow batteries, have been adopted for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another through the disposition of their active materials. Alternately, flow batteries can be considered to decouple power output and energy storage from one another. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or related variants thereof will refer to materials that undergo a change in oxidation state during operation of an electrochemical cell.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell. The battery is charged or discharged through electrochemical reactions of the active materials that occur inside the cell. The electrochemical reactions result in oxidation or reduction of the active materials during charging or discharging.
State of charge is an important operating parameter for flow batteries and other electrochemical systems utilizing electrolyte solutions. The term “state of charge (SOC)” is a well understood electrochemical energy storage term that will refer herein to the relative amounts of reduced species and oxidized species at an electrode within a half-cell of an electrochemical system. For the negative electrolyte solution, the state of charge is defined by the concentration of the reduced species divided by the total concentration of active materials in the negative electrolyte solution. For the positive electrolyte solution, the state of charge is defined by the concentration of the oxidized species divided by the total concentration of active materials in the positive electrolyte solution. For example, when equal amounts of oxidized and reduced species are present, an electrochemical system has a state of charge of 50%. The state of charge values for the individual half-cells in an electrochemical system are not necessarily equal to one another, and the state of charge for the full cell depends on the state of charge values for individual half-cells. Among other reasons, parasitic reactions (e.g., H2 evolution, H2O oxidation, carbon corrosion, and the like) can occur at one or both electrodes of an electrochemical system to result in an unbalanced state of charge between the two half-cells.
It can frequently be desirable to monitor state of charge in an electrochemical system in order to provide for more reliable and efficient operation. An unbalanced state of charge can produce several detrimental effects during operation of an electrochemical system. For example, an unbalanced state of charge between the electrolyte solutions of a flow battery can lead to mass transport limitations at one of the electrodes, thereby lowering the round-trip operating efficiency. An accurate measurement of state of charge can allow one to determine the degree of cell rebalancing that is needed to restore the flow battery to more optimal operating conditions.
Despite the desirability for knowing state of charge in an electrochemical system, ready techniques for accurately measuring state of charge are presently lacking, particularly in situ measurement techniques. In situ measurements can be desirable in order to preclude changes in the proportion of oxidized and reduced forms of the active materials that can sometimes occur during offline laboratory analyses (e.g., exposure to air and other like reactive conditions that are not reflective of the electrochemical system's operating environment). One conventional technique for in situ determination of state of charge involves use of an oxidation-reduction probe (ORP). However, the probe output is prone to drift over time, and the accuracy of the state of charge measurement can suffer as a result.
In view of the foregoing, electrochemical systems incorporating ready determination of state of charge and methods associated therewith would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.